1. Field of the Invention
This invention relates to a three-dimensional ultrasound image processing apparatus and a coordinate transformation method used in the apparatus, and more particularly to a three-dimensional ultrasound image processing apparatus which forms a three-dimensional ultrasound image based on three-dimensional image data obtained by scanning a three-dimensional region and a coordinate transformation method used in the apparatus.
2. Description of the Prior Art
Three-dimensional image processing apparatuses include various kinds of apparatuses such as a three-dimensional ultrasound image processing apparatus and a three-dimensional X-ray image processing apparatus. A three-dimensional ultrasound image processing apparatus has the function of acquiring three-dimensional echo data and the function of processing thus acquired three-dimensional echo data (hereinafter, the "three-dimensional echo data" is also referred to simply as "three-dimensional data").
More specifically, according to the three-dimensional image processing apparatuses, a scanning plane (two-dimensional data acquiring region) is formed by electronic scanning using an array transducer having a plurality of transducer elements, and a three-dimensional data acquiring region (space) is formed by successively forming numbers of such scanning planes with displacing (pivotally swinging or rotating) the array transducer. The three-dimensional echo data obtained from this three-dimensional data acquiring region is sent to a three-dimensional image processing circuit via a signal processing circuit. Then, by reconstructing the three-dimensional echo data, a three-dimensional ultrasound image for the three-dimensional data acquiring region is formed. Such a processing is also performed in a three-dimensional X-ray image processing apparatus.
Now, in a conventional three-dimensional ultrasound image processing apparatus, each of echo data for the three-dimensional echo data acquiring region is temporarily stored in a three-dimensional echo data memory which has a storage region corresponding to the three-dimensional data acquiring region. In this case, each of the echo data is represented by the three-dimensional polar coordinates in which each data is defined by the depth r of the data on the ultrasound beam, the angle .theta. of the ultrasound beam relative to the scanning start position (beam) in the scanning plane and the displacement angle .PHI. of the scanning plane. Therefore, when the echo data is to be stored in the three-dimensional echo data memory, each of echo data is subjected to a coordinate transformation by which the three-dimensional polar coordinates are transformed into the three-dimensional orthogonal coordinates (X, Y, Z). In other words, each of echo data is stored in an address of the memory specified by the three-dimensional orthogonal coordinates, and thus stored echo data will be reconstructed to form the three-dimensional image.
However, such transformation of the three-dimensional polar coordinates into the three-dimensional orthogonal coordinates requires a large amount of arithmetic operations, and such an increased operational amount causes a problem in that it takes a long time for processing the data. In this connection, it is naturally preferable to make the time from data acquisition to image display as short as possible, and if feasible, it is desired to display the three-dimensional image in real time. In order to resolve such a problem, it may be possible to reduce the time for arithmetic operations through the use of a high speed processor or the like. In such a case, however, there arises other problem in that the cost for the image processing apparatus is markedly increased.
Moreover, in forming such a three-dimensional ultrasound image, it is necessary to carry out interpolation between ultrasound beams (lines) and interpolation between scanning planes (frames). However, execution of these interpolations for the data for the three-dimensional coordinate space also requires, a large amount of arithmetic operations as stated in the above. This also makes it difficult to shorten the time for arithmetic operations.
Further, it is to be noted that the above problems are not limited to the case of a three-dimensional ultrasound image processing apparatus, and other three-dimensional image processing apparatuses also involve such problems.